Cordless radio telecommunication systems, such as operating in accordance with the Digital Enhanced Cordless Telecommunications (DECT) or the Personal Handyphone System (PHS) standards and microcell cellular mobile radio communication systems operating in accordance with the Global System for Mobile communication (GSM) standard, for example, comprise a plurality of radio access units, each providing service to a relatively small area, generally having a radius from 10-100 m indoor and up till 5000 m for outdoor usage.
The radio access units are operatively connected to a central interface unit which, in turn connects to a Private Automatic Branche exchange (PABX), a Public Switched Telephone Network (PSTN) or an Integrated Services Digital Network (ISDN), for example.
Between the radio access units and remote subscriber units, such as a portable radio telephone, a duplex radio link can be established. While moving across the coverage or service area of the radio telecommunication system, the radio link is handed over from one radio access unit to the other if the remote unit leaves the coverage area of a particular radio access unit.
To allow handover of a call in progress from one radio access unit to the other without interruption and not requiring special equipment in the remote subscriber unit, it is required that the radio access units at their air interface operate synchronously. To this end, the radio access units receive relevant timing information from the central interface unit.
In practice, however, the radio access units connect to the central interface unit via transmission paths of different lengths. This, because some of the radio access units will be physically located further away from the central interface unit than others. Those skilled in the art will appreciate that timing information generated by the central interface unit and transmitted to the radio access units, due to propagation time delays at the transmission paths between the central interface unit and the radio access units, will arrive at the different radio access units at different points in time. Accordingly, compensation of such time delays is required in order to achieve the required synchronous operation at the air interface.
In a cordless radio telecommunication system presently available from applicant, after installation, the transmission time delays of the several transmission paths between radio access units and the central interface unit are measured separately and are manually entered into the system. In the case of very small systems, if the radio access units are connected by cable to the central interface unit, for example, cables of fixed length can be used, the transmission time delays of which are known and can be entered into the system.
This known method has a number of disadvantages. First, special equipment and trained personal are required for establishing the different time delays and entering same into the system. This can be a very time consuming task, in particular in the case of large systems wherein up till 120 radio access units can be connected to a signal central interface unit. Further, errors in the measurement of time delays or wrongly entered values will only be detected from complaints about the communication, once the radio telecommunication system is in operation. This of course being very annoying to users. Further, the radio access units, after their installation, can not be immediately connected to the transmission cable, or other transmission medium, because first the transmission time delay has to be established. After establishing the transmission delays, an extra step is required in connecting the several radio access units to their transmission cable, for example, which adds extra costs to the installation of the system.
Several attempts have been made to avoid the above unreliable and laborious procedure.
European patent application 0.626.796 discloses a method for synchronizing radio access units or base stations in a multi-cell cordless radio telecommunication system, wherein adjacent base stations are synchronised in a master slave relationship. The radio base stations connect to a central interface unit.
Starting from a first base station, under the control of the central interface unit, a synchronisation signal is transmitted from the first base station for receipt by adjacent base stations. A base station which receives the synchronisation signal from the first base station generates a receive signal and transfers same to the central interface unit. The receive signal comprises information concerning the signal strength level of the received synchronisation signal. The central interface unit evaluates the signal strength levels of the base stations which have received the information signal, if any, and sends a command to a second base station, reporting the highest signal level, for starting a synchronisation procedure at this second base station, to synchronise its transmissions to the transmissions of the first base station.
The second base station next transmits a synchronisation signal for receipt by its adjacent base stations, which will cause the generation of receive signals at other base stations. From an evaluation of the reported signal strength levels, the central interface unit will select a third base station to synchronise its transmission to the transmission of the second base station etc.
At the end of the procedure, at the air interface, base stations are mutually synchronized, however there is no global or common synchronisation between all the base stations at the air interface.
European patent 0.437.835 discloses a frame signal synchronisation system for multiple radio base stations in a Time Division Multiple Access (TMDA) digital mobile communication system, wherein the base stations are provided with delay adjustment means at the link between the radio base stations and the control station or central interface unit to which the base stations connect.
Synchronisation between the base stations is achieved by transmitting a reset pulse from the control station to the base stations, for resetting the TDMA frame generators. To this end, the control station is equipped with synchronous signal generating means, having a delay processor. The delaying means of the base stations are provided with returning means for receiving the reset pulse and returning it immediately to the synchronous signal generating means as a return pulse. The delay processor of the synchronous signal generator means detects the time when the return pulse is received and, on the bases of a period of time defined by the time when the reset pulse is transmitted and the time when it is received, determines a time delay of the delaying means of a particular base station.
Although, at the air interface, a global synchronisation of the base stations of the entire system is achieved, the system according to this European patent comprises a huge signalling overhead, because a reset pulse is send every TDMA frame and the base stations have to be equipped with delaying means capable of transmitting return pulses. In particular in the case of multi-cell radio telecommunication systems wherein hundred or even more radio access units or base stations connect to a central interface unit or control station, a powerful delay processor is required in order to calculate the several link transmission delay times and transmitting same to the relevant delaying means.
In the article "Autonomous Decentralized Inter-Base-Station Synchronisation for TDMA Microcellular Systems", by Yoshihiko Akaiwa, et al., published in CH2944-7/91/0000/0257, 1991, IEEE, p. 257-262, a method for the synchronisation of multiple base stations in a TDMA radio telecommunication system is disclosed, wherein a base station monitors TDMA signals from other base station to measure timing errors and received power levels. The timing errors, which are defined as differences in timings between other base stations and the measuring base station, are averaged with the received power levels as weighing factors. The result is used to correct the timing of the measuring base station. This process is repeated periodically, and is performed at each base station.
With this method, local synchronisation between adjacent base stations can be achieved, however no global synchronisation between all the base stations at the air interface. Further, base stations have to be provided with processing power in order to perform the averaging process, which unduly will raise the costs of such base stations.
European patent application 0.560.079 discloses a method of synchronisation of several base stations from time delays measured over the radio path between adjacent base stations. In order to provide a global (air link) synchronisation, the timing of the base stations is adjusted by a timing adjustment which is for each base station evaluated from estimates of the synchronisation delays between the base stations.